Historically, the co-stimulatory molecule B and T Lymphocyte Attenuator (BTLA) has been classified as an inhibitory molecule since its absence enhances immune responses. BTLA belongs to the immunoglobulin superfamily, and it binds Herpes Virus Entry Mediator (HVEM), a member of the TNF receptor superfamily. Interestingly, this unusual receptor-ligand pair has the potential for bi-directional signaling. BTLA-HVEM interactions as well as BTLA polymorphisms, are conserved in mice and humans arguing for its relevance in modulating immune responses. A human BTLA polymorphism has been linked to rheumatoid arthritis susceptibility. It is likely this polymorphism affects BTLA in the inhibitory region of the cytoplasmic domain, which could potentially tip the balance towards a pathological immune response. Our preliminary data supports the inhibitory action of BTLA with two in vivo models, MRL/lpr and experimental allergic encephalitis (EAE) models, where the disease is accelerated in the absence of BTLA. However, an unexpected observation was made in a graft-vs-host disease model where the absence of BTLA results in reduced survival of alloreactive donor cells. Therefore, the presence of BTLA can either inhibit immune responses or promote cell survival. Furthermore, the cytoplasmic domain of BTLA possesses several functional domains, two ITIM motifs work cooperatively to recruit SHP1 and SHP2 and are thought to mediate the inhibitory signals. Also, a conserved Grb2 site can recruit PI3K P85, which may allow for pro- survival signals. Therefore, BTLA has both positive and negative signaling potential. The overall goal of this proposal is to test the hypothesis that BTLA engagement can lead to bi-directional signals;those that are cell-intrinsic (into the cell expressing BTLA) or cell-extrinsic (through the binding of BTLA to HVEM) explaining the inhibitory and pro-survival actions of BTLA. Aim 1 will use the MRL/lpr and graft-vs.-host disease models to discriminate cell-intrinsic from cell-extrinsic actions of BTLA in vivo. Aim 2 will focus on the cell-intrinsic signals of BTLA by analyzing the ITIM and Grb2 cytoplasmic signaling motifs in vitro and in vivo. Co-stimulatory molecules help to balance the immune system so that diseases can be eliminated, without causing harm to the body, which occurs in autoimmune disorders. The co-stimulatory molecule B and T Lymphocyte Attenuator (BTLA) typically inhibits immune responses through binding with its partner HVEM. Recently a mutation of BTLA was linked to rheumatoid arthritis susceptibility and very little is known how this occurs. Within this proposal we hope to discover specific targets of intervention for autoimmune disorders.